1. Field of the Invention
The present invention relates to an image sensor where pixels having photoelectric conversion elements are arranged in a matrix, and more particularly to an image sensor which can improve image quality of an image which is partially bright, such as sunlight.
2. Description of the Related Art
An image sensor, such as a CMOS image sensor, has photoelectric conversion elements at pixels, converts the intensity of light which enters during a predetermined integration period into electric signals, performs image processing, and outputs image signals. When a row select line is driven, the photoelectric conversion signal of the pixel connected to the row select line is held by a sample hold circuit which is disposed in each column, and this detection signal being held is sequentially output according to the horizontal scanning pulse.
Such a CMOS image sensor is disclosed, for example, in Japanese Patent Laid-Open No. 2002-218324.
A pixel of a conventional image sensor is comprised of a photodiode which is a photoelectric conversion element, a reset transistor for resetting cathode potential thereof, an amplifying transistor for amplifying the cathode potential, and a selecting transistor, for example. After the cathode potential of the photodiode is reset to the reset potential, the decreased cathode voltage, which is decreased by the current which the photodiode generates according to the received light intensity during a predetermined integration period, is held by the sample hold circuit as the photoelectric conversion voltage.
Reset noise is superimposed on the cathode potential at reset. This reset noise is different depending on the pixel since reset noise depends on the dispersion of the transistor characteristics and the dispersion of the parasitic capacitance in the pixel. To remove the reset noise from the detected voltage, the sample hold circuit is comprised of a correlative double sampling circuit. The correlative double sampling circuit sample holds the cathode potential at the completion of the integration period after the first reset operation, and after the second reset operation being executed immediately after this, sample holds the reset noise, which is generated after the subsequent reset noise read period. And the differential voltage of the two sample hold voltages is output as the pixel signal. Reset noise is removed from the detected signals by determining the difference of the two sample hold voltages.
However, when an area where brightness is much higher than the surrounding images, such as sunlight, exists locally in the captured image, the photoelectric conversion current generated by the photodiode becomes extremely high in the corresponding pixel. This higher brightness rapidly decreases the cathode potential during the reset noise read period after the second reset operation. As a result, the differential voltage of the two sample hold voltages becomes very small. In other words, the pixel signal level to be detected in an area where brightness is high, such as sunlight, is supposed to be the maximum level, but the expected pixel signal level cannot be obtained because the reset noise read at the end of the reset noise read period becomes too high. As a result, the output image to be obtained becomes an image where the brightness of the sun is extremely low, and the sun becomes completely black in an extreme case.